The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a carrier head for a chemical mechanical polishing apparatus.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, the layer is typically etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly non-planar. This non-planar surface presents problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing pad. The polishing pad may be either a "standard" pad or a fixed-abrasive pad. A standard pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles, if a standard pad is used, is supplied to the surface of the polishing pad.
The carrier head usually includes a retaining ring. The retaining ring is positioned around the substrate to ensure that the substrate is held in a recess beneath the carrier head during polishing. The retaining ring may be affixed directly to the carrier head, or it may be connected to the carrier head by a flexible connector, such as a flexible membrane or bellows.
To conduct polishing, a substrate is loaded into the carrier head and positioned by the carrier head against the polishing pad. The loading operation typically occurs at a transfer station which includes centering equipment to align the substrate with the recess defined by the retaining ring.
One problem that has been encountered in CMP is that, in some carrier head designs, the retaining ring is free to pivot about a point located above the polishing pad surface. The pivoting action can lift one side of the retaining ring and lower the other side. This creates an uneven pressure distribution on the polishing pad, reducing the polishing uniformity.
Another problem is that the retaining ring needs to be periodically replaced. However, the retaining ring may be difficult to replace, requiring complete disassembly of the carrier head.